This application is a 35 USC 371 application of PCT/DE 99/01664 filed on Jun. 08, 1999.
1. Field of the Invention
This invention is directed to an improved pump assembly, and more particularly to an improved high-pressure pump for a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One pump assembly of the type with which this invention is concerned is already known from German Patent Disclosure DE 195 49 108 A1, in this known pump the flow control valve, in its position that blocks the supply flow to the high-pressure pump, diverts the fuel, brought from the low-pressure pump, to the fuel tank. A flow control valve of the slide type has a more or less major leakage flow, because of its design. In the xe2x80x9czero pumpingxe2x80x9d operating state of the high-pressure pump, however, the aspiration of the leakage flow by one or more of the pump elements of the high-pressure pump should be avoided. This requires setting the suction valves of the high-pressure pump to a relatively high opening pressure, with the disadvantageous consequence that in the xe2x80x9cpartial fillingxe2x80x9d operating state, opening pressure and flow tolerances lead to a non-uniform filling of the pump elements and thus a non-uniform pumping on the part of the high-pressure pump. Varying wear behavior of the suction valves over the service life of the pump can cause further impairment of the equal pumping.
The pump assembly according to the invention, has the advantage over the prior art that the high-pressure pump, in the xe2x80x9czero pumpingxe2x80x9d operating state, does not pump any fuel, since the differential pressure valve is not overcome by the leakage flow of the flow control valve. Accordingly, the suction valves of the high-pressure pump can be set to a very low opening pressure, which in turn assures the uniform filling of the pump elements with fuel in the xe2x80x9cpartial fillingxe2x80x9d operating state. Tolerances and changes in opening pressure over the service life of suction valves set in this way have only slight influence on the equal pumping of the pump elements.